1. Field of the Invention
The present invention relates to a thermal printer and, more particularly, to an improvement in a thermal printer suitably used for preparing a block copy.
2. Description of the Prior Art
A typical conventional thermal printer is shown in FIG. 1. A thermal ink ribbon 3 overlays recording paper 2 wound around a platen 1. The thermal ink ribbon 3 and the recording paper 2 are selectively heated by a thermal head 4 to transfer ink from the ink ribbon 3 to the recording paper 2. In order to print a halftone image in a thermal printer, an image 5, divided into 1024.times.512 picture elements, is formed by scanning with the thermal head 4 having 512 heating elements 6 in the direction V indicated by the arrow shown in FIG. 2. In this case, the thermal head 4 is intermittently moved relative to the paper 1024 times for completing an image. The heating elements 6 are seleotively energized and heated for periods of time corresponding to the image densities of the picture elements. The elements 6 are intermittently stopped to print a line extended in the V direction (which is here-in-after referred to as a V line). It should be noted the head 4 in the printer of FIG. 1 is fixed, and that the platen 1 is intermittently rotated to perform the required scanning.
In order to reproduce a full-color image, four ink ribbon sheets such as Y (yellow), M (magenta), C (cyan), and B (black) color ink ribbon sheets are used, and scanning is performed one color at a time. In some case, B (Black) color printing may be omitted.
The above conventional printer is disclosed in U.S. Pat. No. 4,496,955.
In the field of full-color printing, when four block copies, i.e., Y, M, C, and B copies are prepared from a single full-color image, the full-color image is separated by a color scanner to obtain four monochromatic images whose densities respectively correspond to levels of Y, M, C, and B color components. These monochromatic images are converted into dot pictures to prepare the corresponding block copies.
The above method requires an expensive color scanner, and color separation is also cumbersome and time-consuming.